“An art forgery investigator deciphers location of stolen diamonds using her brother’s diary that mysteriously appears at her doorstep. She embarks on a hunt for the treasure finding her family lost decades ago and the truth about her father who masterminded an international heist.”
The inciting incident is when she finds the diary at the doorstep, right?
Perhaps rewrite the logline using the formula recommended here: How to write a logline.
I think you have the goods for an engaging story. Now refine the logline!
I hope this helps.